Recent interest in the use of hyperthermia in cancer therapy has brought about a need for increased understanding of its effects on cells. Earlier workers postulated that cell lipid viscosity was the determining factor in a cell's response to temperatures. The studies proposed will undertake to systematically study the effect of varying membrane lipids, cell microviscosity, and survival at elevated temperatures. An auxotroph of E. coli (K1060) which must be provided unsaturated with membranes of differing lipid composition and properties. Cell microviscosity will be studied using the polarization properties of a fluorescent probe molecule dissolved in the membrane. Cell survival will be determined on cells held at a graded series of temperatures. These data will be modeled to provide an estimate of the relative role of each factor in hyperthermic killing.